My system was starting to behave strangely (Windows closing and opening all by themselves), so I thought about this final update (reminds me of Star Trek - The Final Frontier ).
I managed to Boing Bag my AmigaOS system after re-installing the XAD library ( http://aminet.net/package/util/arc/xadmaster000 ). If you still have difficulties, just make sure :

The Amiga OS 3.9 CD is inserted

You don't use the XAD update included

Boing Bag 1 is installed

You don't mess with the files inside BB2

You don't update the ROM if you run AmigaOS from WinUAE

The author insisted upon that : XAD is not freeware , but it's fully functional.

By the way, some developer is working on an unofficial Boing Bag 3. Nice, isn't it ?